The present applicant has invented an “electric heater” (Patent Document 1) that forms a resistive element such as a platinum film on a film floating in midair and uses it as a heater, and this electric heater is applied as a micro heater for a flow sensor or a vacuum sensor at present. Further, the present applicant has also invented “heated diode temperature measuring apparatus, infrared temperature measuring apparatus and flow measuring apparatus using this apparatus, and method for manufacturing flow sensing unit” (Patent Document 2). Furthermore, since a semiconductor diode can be also used as a temperature sensor, the present applicant has suggested using the semiconductor diode as a heater and a temperature sensor. Then, the present applicant has also invented “thermocouple heater and temperature measuring apparatus using this heater” (a thermocouple heater) that can use a thermocouple as not only a temperature difference sensor but also a heater (Patent Document 3).
There has been conventionally a gas flow sensor as a heat conduction-type sensor which has a configuration bridging a groove-shaped flow path (a cavity) formed in a substrate, has three thin-film bridges (bridges made of thin films floating in midair) thermally separated from this substrate being symmetrically formed along the groove, has a platinum film formed on each thin-film bridge, uses the central thin-film bridge as a heater, and also uses the thin-film bridges on both sides as temperature sensors (Patent Document 4). Temperatures of the thin-film bridges on both the sides symmetrically provided with the central heater at the center have the same temperature when there is no flow of a gas along the flow path, but the thin-film bridge on the upstream side is cooled since a cold gas having a low ambient temperature flows thereto whilst the downstream side receives heat from the central heater thin-film bridge to increase the temperature when there is a flow of gas. As described above, since a temperature difference is produced between the thin-film bridges on both the sides of the heater due to a flow of the gas, this is a method for measuring a gas flow by utilizing this matter. However, a resistor temperature sensor such as a platinum thin-film is an absolute temperature sensor, and hence resistance itself is associated with an absolute temperature. Therefore, for example, since two absolute temperature sensors must be prepared in order to measure a temperature difference and a difference between outputs from these sensors must be obtained, this method is inappropriate to measure a fine temperature difference because of a large error.
Furthermore, in a conventional thermal flow sensor, a platinum thin film is used as a heater and an absolute temperature sensor in many cases. Therefore, since a variation with time is large and effects of an ambient temperature are reflected as they are since this sensor is an absolute temperature sensor, correcting the ambient temperature is difficult, many sensors and a temperature control system using these sensors are required to enable this correction, and the flow sensor necessarily becomes expensive.
In general, when a thin film floating in midair is used for thermal separation from a substrate, the heated thin film is subjected to heat radiation and cooled based on Newton's law of cooling in proportion to a temperature difference (T−Tc) between a temperature of the substrate which is an ambient environmental temperature Tc (an ambient temperature before being heated) and a temperature T of the heated thin film at the time of stopping heating, and the temperature of the thin film eventually becomes equal to the temperature of the substrate. In this manner, a temperature of a heated object is thermally conducted to a surrounding medium, and the temperature is increased or decreased in relation to a heat transfer coefficient of the surrounding medium. In the heat conduction-type sensor used for measuring a change in temperature of a temperature sensor to measure a physical state, e.g., a flow velocity, a mass flow, a degree of vacuum, an atmospheric pressure, and others of a surrounding medium, a temperature difference between a temperature of a substrate, which can be considered to as an ambient temperature Tc, and a temperature T of a heated thin film is more important than an absolute temperature. In this manner, to measure a temperature difference, a thermocouple or a thermopile as a small temperature-difference sensor that outputs a temperature difference only is more preferable than an absolute temperature sensor such as a platinum resistive element or a thermistor since it can perform measurement while being hardly affected by a change in ambient temperature.
The present inventor has invented an impurity concentration sensor as a gas flow sensor using a thermocouple which is a temperature difference sensor on ahead (Patent Document 5). Moreover, the present inventor has also invented a sensing unit and a thermal flow sensor having this unit mounted thereon as a temperature sensor that can measure a temperature difference by using a thermocouple on ahead (Patent Document 6). In this thermal flow sensor, a cavity is extended in a flow direction of a fluid of a heater, and the temperature sensor or the heater is formed on an SOI thin film that protrudes in a cantilever shape with a side surface of a substrate parallel to the flow direction of the cavity being used as a supporting portion. Additionally, although the cantilever-shaped thin film of the heater is arranged at the center, and thermocouples are formed into a similar cantilever-shaped thin film and arranged on an upstream side and a downstream side of the flow on both sides in close proximity, the cantilever-shaped thin film of the heater and the thin film having the temperature sensor formed thereon are not coupled with each other excluding the supporting portion, and hence heat from the cantilever-shaped heater (the thermocouples can be also used as heater in this example) is thermally conducted only through a surrounding fluid such as a gas which is a measurement target fluid to heat the thin films having the temperature sensors arranged on the upstream side and the downstream side on both sides of the heater. Therefore, the cantilever-shaped thin film having the temperature sensor on the upstream side is cooled by a flow of the fluid, and the cantilever-shaped thin film having the temperature sensor on the downstream side receive heat from the heater to increase a temperature, thereby measuring a flow of the fluid with high sensitivity based on a temperature of the fluid alone. However, there is a problem that a heat transfer coefficient of the fluid varies when a temperature of the surrounding fluid changes and, although the thermocouples as temperature difference sensors are provided as the temperature sensor, a temperature difference output differs due to a fluctuation of temperature of the fluid itself even when the heater is heated with the same power or the same is heated while controlling to provide a temperature difference from the same substrate because of temperature dependence of the heat transfer coefficient of the fluid, thereby complicating temperature correction. Of course, when a type of the fluid is changed, its heat transfer coefficient differs, and hence there is a problem that effects of the type of the fluid are also considerable.
There has conventionally been a flow sensor which has a thermopile provided on the same thin film formed in the same cavity and has heaters symmetrically formed and arranged on the upstream side and the downstream side to measure only a difference between preceding and subsequent temperatures of a heater generated due to a flow of a fluid, whereby temperature dependence of a heat transfer coefficient of the fluid is finally reduced (Patent Document 7). However, these examples have a problem that a region of the sensor portions is necessarily increased since the thermopile is used to amplify a temperature difference output, a diaphragm structure must be provided to increase a junction area with respect to the substrate in order to form many cold junctions, whereby heat from the heater flows to the substrate side to reduce the temperature and a temperature sensor region has a temperature distribution. Further, a flow sensor that has a heater at the center, and thermocouples arranged in a radial pattern or a temperature-sensitive resistive element thin film being arranged around the heater has been also conventionally reported (Patent Document 8), but a thin film is extended along a flow direction of a fluid and a supporting portion is also provided in the flow direction, whereby movement of heat is promoted in the flow direction of the fluid to form a temperature distribution. In particular, when using thermocouples as a temperature sensor, there is a problem that heat that moves along the flow of the fluid is conducted to the substrate side in the flow direction to facilitate cooling and a position of a peak in the temperature distribution may possibly get across a position of a hot junction of the thermocouples.
In general, when measuring a physical quantity (which represents physical properties such as density, thermal conductivity, or specific heat inherent to a measurement target fluid here) or a physical state (which represents an adjustable amount such as a flow velocity, a mass flow or an atmospheric pressure of the measurement target fluid here) of the measurement target fluid, a sensor that transfers heat from a heater having heat to a temperature sensor based on heat conduction through the measurement target fluid to grasp the physical quantity or the physical state of the measurement target fluid based on an output from the temperature sensor is called a heat conduction-type sensor.
Therefore, a thermal flow sensor or a thermal barometric sensor according to the present invention also falls within the category of the heat conduction-type sensor and has common points as the heat conduction-type sensor. In conventional technologies, when using the thermal flow sensor to measure a flow velocity or a mass flow of a measurement target fluid, a type of the measurement target fluid must be known in advance. In such a situation, in case of a specific standard measurement target fluid (a standard gas), e.g., a gas, when an output value such as a flow velocity or a mass flow of the thermal flow sensor which is indicated by utilizing basic data of a nitrogen gas such as a degree of thermal conductivity at 20° C. and 1 atmosphere is changed since a type of the measurement target fluid is different from the standard gas, the output value, e.g., a flow velocity or a mass flow of the thermal flow sensor is calibrated using known basic data such as a degree of thermal conductivity of the measurement target fluid at a temperature or an atmospheric pressure of the measurement target fluid. In measurement of a mass flow (e.g., a mass flow rate) of a helium gas or a hydrogen gas having a high degree of thermal conductivity, the indication largely deviates from that of the nitrogen gas as the standard gas, and calibration must be necessarily performed. This situation is the same in regard to the thermal barometric sensor as the heat conduction-type sensor, a gas type of a measurement target fluid must be known in advance and an output value must be calibrated as long as a heat transfer coefficient of the measurement target fluid concerns the measurement. As described above, the heat conduction-type sensor that can perform automatic calibration without knowing a gas type of a measurement target fluid in advance has been demanded.